Models view threshold

[Relfos]

Added minimum and maximum viewing distance to camera in order to optimize display conditions of models

[pjcozzi]

Thanks @Relfos but we are going to implement this using more generic "display conditions" so we can support different conditions like viewer altitude, number of pixels, etc. You should be able to use the display-condition branch for now, e.g.:

var model = Model.fromGltf({
  url : uri,
  displayCondition : new DistanceDisplayCondition(0, 1000.0) // Only draw if center is < 1,000 meters away
});

Contributions to that branch are welcome. This is for #758.

I'll look at #2177 now.